Natural gas consists of a mixture of lower gaseous hydrocarbons occurring in the earth's crust. It frequently exists near crude oil deposits and is invariably mixed with, dissolved in, or otherwise associated with sizeable amounts of petroleum impurities. Other impurities, particularly water and various salts, are also frequently present in significant proportions. Thus freshly extracted natural gas presents formidable cleanup problems before it can ultimately be placed in a condition suitable for use by the consumer.
Natural gas is processed in conventional gas refining plants before being piped over long distances to various distribution points or use locations. For example, a main transporting pipeline may carry natural gas toward a city or other municipality until reaching a point wherein gas is tapped off through a reducing valve to travel through a secondary pipeline to its ultimate destination, storage facility, etc.
It is desirable that no water vapor be admitted to the transporting pipeline. Quantities of water or moisture that are relatively small compared to the quantities initially present when the gas is extracted from the earth may freeze and block the pipeline such that flow is completely halted or at least greatly restricted.
Additionally, freshly extracted natural gas may contain sizeable amounts of salts which are mostly in the form of sodium chloride and iron sulphide, but which are also present as other species. The corrosive action of salts on pipes and other facilities is well known to the gas-refining industry. Thus, the removal of water and salts from natural gas is commonly an integral function of any gas processing plant.
The means used nearly universally to accomplish the removal is a large tank equipped with baffles called an absorber. Natural gas arriving from many wells is admitted into the bottom of the absorber and upflows toward the top. At the same time a glycol, such as diethylene or triethylene glycol, is admitted continuously into the top of the tank and trickles downwardly over the baffles in countercurrent exchange with the upflowing gas. The net result is that the water and salts in the gas are exposed to and preferentially partition into the more polar glycol such that the gas exiting at the top of the absorber is substantially free of these contaminants.
Left behind, however, is a quantity of glycol typically running into the thousands of gallons which may be fouled with sizeable quantities of water, salts such as sodium chloride and iron sulphide, and oil or other crude petroleum contaminants. Water and salt-laden glycol is conventionally pumped through a closed-loop (of which the absorber is part) including various filters, strippers, heat exchangers, etc. and a reboiler wherein the glycol is conventionally heated and maintained at a temperature of from about 250.degree. to about 400.degree. F. such that the water quickly distills off. The glycol may then be returned through the remaining portion of the loop back to the absorber, again to flow in countercurrent exchange with contaminated natural gas.
A very troublesome problem arises herein, however, in that, although the reboiler can easily drive off water from the glycol, the salts and much of the crude oil contaminants remain therein. At times it may take but several trips through the loop before the glycol reaches its saturation point with respect to the salt. Salt starts to crystallize out and settle onto the heating pipes in the reboiler and forms a heat-insulating layer therein, and starts generally to foul the transport piping associated therewith. Ultimately the reboiler becomes so encrusted or corroded that the entire gas processing operation must be shut down and the reboiler must be dismantled and cleaned or repaired. The economic implications of a shutdown are so obvious as not to require description, in addition to which thousands of gallons of glycol must be discarded at a replacement cost close to four dollars per gallon.
Moreover, freshly extracted natural gas is frequently preliminarily stored in so-called "salt domes", i.e. giant cavities which have been artifically eroded into salt deposits by means of water and steam to depths of thousands of feet and diameters on the order of miles. The gas stored in these giant sodium chloride containers picks up even more salt and accordingly exacerbates the down-time problems at gas processing plants when it is re-extracted and processed.
Additionally, problems stemming from the presence of salts in natural gas is especially prevalent in offshore deep sea gas drilling operations. Here, a pipeline would be sunk through a few hundred feet of sea water before drilling through the floor for distances typically on the order of twenty thousand feet. After a "strike" has been made, gas at natural pressures carries the sea water head with it or otherwise absorbs large amounts of water and salt, all of which must be removed in refining operations.
Thus, the presence of salt in natural gas creates technological problems to which no satisfactory solution has yet been discovered. Clearly, a method of cleaning glycols which circumvents such thorny problems would be enthusiastically embraced by the petroleum-natural gas industry. Such a method would employ an apparatus to clean glycols which would be capable of feasibly being performed on-site and which would not require that the entire processing operation be shut down. Such a method and apparatus is the subject of the present invention.
Techniques for cleaning salts from various fluids have been known. For example, ion exchange resins have been used in the prior art to clean dissolved salts from various liquids. U.S. Pat. No. 3,615,924 to Reents discloses a method of purifying a relatively dilute aqueous glycerine solution, which method includes passing the solution through both cation and anion exchange resins. U.S. Pat. Nos. 3,252,897 to Hesler et al and 2,772,237 to Bauman also disclose the use of ion exchange resins to remove ionic impurities from a variety of liquids, including glycols, and are similar in that both disclose using a cation exchange resin in conjunction with a sulfated anion exchange resin to remove a strong acid or salt thereof from a series of liquids.
However, none of these patents contains any suggestion that strong acid cation (in the hydrogen ion form) and strong base anion (in the hydroxide ion form) exchange resins may be used in conjunction to clean relatively concentrated glycols, i.e. those solutions containing at least 80% glycol by volume, associated with the cleaning of natural gas. Nor do any of these patents suggest that ion exchange resins could be used to clean anything contaminated with the amount of petroleum impurities along with the water, as the glycol used in cleaning natural gas. It is not at all clear from these patents how an ionic resin could be integrated into any cleaning apparatus and method used to clean glycols fouled in this manner, it having been thought that the crude oil impurities and the glycol itself would foul the resin such that its use would be foreclosed in any cleaning operation involving glycols.